Ultraviolet and infrared laser based systems and methods are known for enabling ophthalmological surgery on the external surface of the cornea in order to correct vision defects. These procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of an anterior stromal tissue from the cornea to alter its refractive power. In ultraviolet laser ablation procedures, the radiation ablates corneal tissue in a photodecomposition that does not cause thermal damage to adjacent and underlying tissue. Molecules at the irradiated surface are broken into smaller volatile fragments without heating the remaining substrate; the mechanism of the ablation is photochemical, i.e. the direct breaking of intermolecular bonds. The ablation removes a layer of the stroma to change its contour for various purposes, such as correcting myopia, hyperopia, and astigmatism. Such systems and methods are disclosed in the following U.S. patents and patent applications, the disclosures of which are hereby incorporated by reference: U.S. Pat. No. 4,665,913 issued May 19, 1987 for "METHOD FOR OPHTHALMOLOGICAL SURGERY"; U.S. Pat. No. 4,669,466 issued Jun. 2, 1987 for "METHOD AND APPARATUS FOR ANALYSIS AND CORRECTION OF ABNORMAL REFRACTIVE ERRORS OF THE EYE"; U.S. Pat. No. 4,732,148 issued Mar. 22, 1988 for "METHOD FOR PERFORMING OPHTHALMIC LASER SURGERY"; U.S. Pat. No. 4,770,172 issued Sep. 13, 1988 for "METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. Pat. No. 4,773,414 issued Sep. 27, 1988 for "METHOD OF LASER-SCULPTURE OF THE OPTICALLY USED PORTION OF THE CORNEA"; U.S. patent application Ser. No. 109,812 filed Oct. 16, 1987 for "LASER SURGERY METHOD AND APPARATUS"; U.S. Pat. No. 5,163,934 issued Nov. 17, 1992 for "PHOTOREFRACTIVE KERATECTOMY"; U.S. patent application Ser. No. 08/368,799, filed Jan. 4, 1995 for "METHOD AND APPARATUS FOR TEMPORAL AND SPATIAL BEAM INTEGRATION"; U.S. patent application Ser. No. 08/138,552, filed Oct. 15, 1993 for "METHOD AND APPARATUS FOR COMBINED CYLINDRICAL AND SPHERICAL EYE CORRECTIONS"; and U.S. patent application Ser. No. 08/058,599, filed May 7, 1993 for "METHOD AND SYSTEM FOR LASER TREATMENT OF REFRACTIVE ERRORS USING OFFSET IMAGING".
The technique for increasing the curvature of the corneal surface for hyperopia error correction involves selectively varying the area of the cornea exposed to the laser beam radiation to produce an essentially spherical surface profile of increased curvature. This selective variation of the irradiated area may be accomplished in a variety of ways. For example, U.S. Pat. No. 4,665,913 cited above discloses the technique of scanning the region of the corneal surface to be ablated with a laser beam having a relatively small cross-sectional area (compared to the optical zone to be ablated) in such a manner that the depth of corneal removal increases with distance from the intended center of ablation. This is achieved by scanning the beam more times over the deeper regions than the shallower regions. As pointed out in U.S. Pat. No. 5,163,934, such ablations tend to be rougher than area ablations. The result is a new substantially spherical profile for the anterior corneal surface with maximum depth of cut at the extreme outer boundary of the optical zone. Another technique disclosed in the above-cited U.S. Pat. No. 4,732,148 employs a rotatable mask having a plurality of elliptical annular apertures which are progressively inserted into the laser beam path to provide progressive shaping of the laser beam in order to achieve the desired profile.
One of the major difficulties encountered in the application of laser surgery techniques to effect hyperopic refractive error corrections lies in the nature of the boundary between the optical zone and the untreated area. Since the anterior surface of the cornea is sculpted during the process to have an increased curvature, the maximum depth of cut necessarily occurs at the outer boundary of the optical zone. The generally annular region between this outer boundary and the adjacent untreated anterior surface portion of the cornea typically exhibits steep walls after the completion of the photoablation procedure. After the surgery, the tendency of the eye is to eliminate these steep walls by stimulated healing response involving concurrent epithelial cell growth and stromal remodeling by the deposition of collagen, which results in corneal smoothing by filling in tissue in the steep walled region. This natural healing response acts to eliminate the discontinuity, resulting in a buildup of tissue in the steep walled region and over the outer portion of the optical zone. This natural phenomenon, sometimes termed the "hyperopic shift" in phototherapeutic keratectomy, causes a lack of precision for a given surgical procedure and diminished predictability, which tend to counteract the beneficial effects of the refractive correction procedure and thereby reduce the desirability of the procedure to the prospective patient.
In some patients, there are both hyperopia and astigmatism defects in the same eye, requiring correction of both errors in order to improve vision. Astigmatic conditions are typically caused by a cylindrical component of curvature departing from the otherwise generally spherical curvature of the surface of the cornea. Astigmatic conditions are usually corrected by effecting cylindrical ablation about the axis of cylindrical curvature of the eye. These cylindrical ablations tend to increase the sharp transitions in the cornea at the extreme ends of the sculpted area.
What is needed in the field of ophthalmological surgery, therefore, are systems and methods for correcting both hyperopia and astigmatism of the eye by laser removal of the corneal surface. It would be particularly desirable to perform such hyperopia and astigmatism corrections without generating steep walls in the region between the outer boundary of the optical zone and the adjacent untreated anterior surface portion of the cornea.